In recent years, super-clarification techniques for air, gases, water, chemicals, etc. based on the membrane separation techniques have made a remarkable progress and are widely used in various fields such as the fields of foods, semi-conductors and the like. Particularly, a chemicals clarification technique required for fine processing in the process for producing semi-conductor integrated circuits is behind in technical response as compared with the clarification technique for air, gases, water, etc., and it is recently attracting a special attention. As one of the main reasons for that, while increasing the purity of chemicals is being required in attaining an improvement in the yield and quality of products, both semi-conductor integrated circuit manufacturers and chemicals suppliers are behind in their technical response, there is given delay in the development of filter products which are less in metal leaching and good in chemicals resistance. As filters which meet the above requirements at present, ones made wholly of fluorocarbon resins are being required, and membrane filters of a multi-flat membrane system and a tubular form have been developed. But, these filters have not yet come to satisfy the needs of the market, because both effective filter area and flow rate of filtration are so small that there are caused problems in filtration of highly viscous chemicals and mass treatment.
On the other hand, a pleat-form filter element with fluorocarbon resin membranes has been developed. This filter element has a large effective filter area as well as a fairly superior chemicals resistance, but because of the adhesion of fluorocarbon resin being very difficult in itself, parts other than the filter membrane are made of materials other than fluorocarbon resin which are not sufficient in terms of chemicals resistance, temperature resistance and metal leaching.
Consequently, satisfactory products are not yet obtained.
Further, a filter employed in the foregoing process for producing semi-conductor integrated circuits requires pores as very fine as 0.01 to 10 .mu.m. Also, the filter membrane itself is very low in strength, so that severe conditions cannot be employed on filter production, and besides the members of the filter element must be assembled not by mere adhesion, but by strong adhesion in a completely sealed state.
In U.S. Pat. No. 3,457,339 Pall proposes a process for applying end caps to filter tube by liquefying an inside face of a thermoplastic end cap by about 10 to 90% of the thickness of the end cap while cooling the outside face of the thermoplastic end cap. However, though this method is useful for the production of filter element of thermoplastic resin having a low viscosity such as about 50 c.p., it is not applicable to a thermoplastic resin having a high melting point and a high viscosity such as fluorocarbon resin, because if the filter sheet material (filter tube) is inserted into the cap, the inside of the cap cannot be heated, and then the temperature of the sheet is lowered and the viscosity increases to cause the pleats to buckle thereby making it impossible for resin to enter the pleats. Pall, further, suggests in U.S. Pat. No. 4,154,688 the welding of the filter sheet material to an end cap of fluorocarbon resin such as Teflon (polytetrafluoroethylene) according to his prior Patent. However, as stated that these are more difficult to bond, Teflon is not fluid even at the temperature above melting point, so that it can not be welded with filter sheet material according to his prior Patent.
Formerly Miyagi, Inoue, Hamada, Fujimogi and Hori invented a filter element made wholly of fluorocarbon resin and production thereof (Japanese Patent Application No. 167202/1983, U.S. patent application No. 608,309 now U.S. Pat. No. 4,588,464 which issued May 13, 1986). In this application in order to weld pleats with an end cap liquid-tightly the both end parts of the pleats are pre-welded, and then after cooling the welded part is inserted into a melted fluorocarbon resin in a mold to force the resin into the spaces of pleats, after which end parts are fitted into end caps. This method needs the pre-welding of the end part of the pleat.